The present invention relates to the field of magneto-optic recording. More particularly, it relates to improvements in magneto-optic recording elements of the type having a direct-overwrite capability.
Conventional magneto-optic disks require two revolutions of the disk to record information. The first revolution is used to erase any previously recorded information, while the second revolution is used to record the new information. The information is stored as patterns of vertically oriented magnetic domains arranged along an annular track on the magneto-optic disk. Traditional magnetic recording systems have a direct-overwrite capability in that erasure of previously recorded information is intrinsic in the recording process. Hence, magnetic systems require only one revolution to record data. As a consequence, compared to magnetic disks, magneto-optic disks are disadvantageous from the standpoint of requiring a substantially longer effective access time.
In U.S. Pat. No. 4,882,718 to Sheih and Kryder, a method is disclosed for eliminating the above-noted two-revolution requirement. Here, the recording element comprises a single magnetic recording layer, and a laser is switched between two power levels according to the digital information being recorded. The lower power level raises the temperature of the recording layer to a level sufficient to destabilize and collapse existing magnetic domains; this has the effect of erasing existing information. The higher power level acts upon the recording layer to create new domains according to the information being recorded. While theoretically plausible as a solution to the two-revolution requirement, experimental evidence indicates that this process results in poorly formed domains and, hence, a low signal-to-noise (SNR) ratio.
In U.S. Pat. No. 4,855,975 to Saito et al, an alternative technique is described for eliminating the noted two-revolution requirement. Here, the recording element comprises two different magnetic layers or films laminated together. One layer, the reference layer, has a low room-temperature coercivity H.sub.c(ref.) and a high Curie temperature T.sub.c(ref.). The other layer, the so-called memory layer, has a high room-temperature coercivity H.sub.c(mem.) and a low Curie temperature T.sub.c(mem.). As the disk shaped recording element rotates, the disk passes in close proximity to an initialization magnet, thereby exposing the disk to a field -H.sub.i perpendicular to the disk surface, where H.sub.c(mem.) &gt;H.sub.i &gt;H.sub.c(ref.). This initialization field serves to vertically orient all magnetic domains of the reference layer in the a given direction (e.g. "down") but has no effect on the memory layer. A second magnet, the bias magnet, is arranged to expose the area of the disk which is selectively heated by an intensity-modulated laser to a field H.sub.b, where H.sub.c(mem.) &gt;H.sub.c(ref.) &gt;H.sub.b. The bias field H.sub.b is perpendicular to the disk and directed in the direction opposite to that of -H.sub.i. While the disk is being read, H.sub.b has no effect on either layer.
When the above-described disk is exposed to a certain power of laser light, the memory layer will be heated above its Curie temperature, while the reference layer remains below its Curie temperature. Under these conditions, the magnetic exchange interaction which exists between the two layers will cause the magnetization of the memory layer to be aligned with the magnetization of the reference layer. Whenever the disk is selectively exposed to a higher light power, both layers become heated to temperatures above their respective Curie temperatures, and the magnetization of the heated portions of both layers become realigned in the direction of the bias field H.sub.b, i.e., the field produced by the bias magnet. Consequently, by modulating the laser light intensity between these two power levels, digital information can be recorded while simultaneously erasing any pre-existing information.
As noted above, the exchange interaction between the two magnetic layers serves to align the magnetization of both layers. Beginning with the magnetization of the layers aligned, an applied magnetic field must overcome the coercivity of one of the layers plus the exchange force to cause them to be aligned antiparallel. Similarly, starting with the magnetic domains of the respective layers oppositely aligned, an applied field must overcome the coercivity of one layer less the exchange force to cause the layers to be aligned parallel. Unfortunately, the exchange interaction has the effect of causing the apparent coercivities of the two layers to converge, making it difficult, at best, to switch one film independently of the other, as required by this direct-overwrite scheme. An alternative view of this observed effect is that a domain wall must form between the two layers wherever their magnetization is oppositely aligned. The energy stored in this wall makes it unfavorable for the layers to switch independently.
To mitigate the above-identified problem, it has been proposed that a layer of gadolinium-iron-cobalt (GdFeCo) be positioned between the memory and referenced layers. The exchange interaction is now mediated through this intermediate layer. This intermediate layer has little intrinsic anisotropy as opposed to the strong perpendicular anisotropy of the two magnetic layers. Both of these conditions lower the energy of the state where the magnetization of the reference layer is antiparallel to that of the memory layer. This makes it easier to obtain the balance between sufficient coupling to cause the layers to align during the lower power write step and allowing the initialization magnet to switch only the reference layer. Such a multilayered recording element is disclosed in European Patent Application No. 319,004, published June 7, 1989.
In both of the multilayer direct-overwrite magneto-optic recording elements described above, the various layers are comprised of similar materials, but with different concentrations of components. In this situation, components from one layer may diffuse into the adjacent layer. This diffusion is accelerated by the temperature excursions which the magneto-optic layers undergo during each write/erase cycle. Diffusion, over time, will change the magnetic properties of the layers, eventually rendering direct-overwrite impossible. Moreover, the proposed recording elements use rare-earth-transition metal alloys which are highly susceptible to corrosion.